1. Field of the Invention
This invention relates to stretch films having particular use as to overwrap films designed for wrapping large objects, to structures and compositions for such films, to methods for making such films, and to methods of using such films, and most particularly to such films having at least one cling (tacky) surface.
2. Description of the Related Art
In general, overwrap films have been used for packaging various types of goods, from retail foods and cigarettes to carpets and furniture. Industrial overwrap films are primarily designed for wrapping loaded shipping pallets and large articles such as furniture, carpets, books, and the like.
Industrial overwrap films typically are provided as one of two types, which types are either stretched around or shrunk over the object packaged. A stretch overwrap film is used by being stretched around the object and then adhered to itself; the tendency of the film to resist further stretching provides a load containment force. For using overwrap shrink film, the film is wrapped around the object and then shrunk, typically by heating, to provide a load of containment force.
Industrial overwrap films are typically based on a single layer or multilayer film comprising ethylene homopolymer or a copolymer of ethylene. Because of the relative largeness and/or heaviness of the wrapped items, these industrial films require a certain degree of toughness, as well as significant resistance to puncture and tearing. These industrial films should also exhibit dimensional stability: once the article is wrapped, the overwrap film should neither stretch, which would allow the wrapped articles to shift and possibly escape the wrapping, nor shrink, which could damage the wrapped article.
Shrink overwrap films are typically biaxially oriented at an elevated temperature and then cooled in a biaxially stretched state. Shrinkage of the film is promoted upon reheating to the elevated orientation temperature. The shrink overwrap film and overwrap heating apparatus are designed to provide a desired degree of shrinkage during reheating of the film. Accordingly, reheating the film causes shrinkage, so that the shrinking film encases the wrapped articles. The degree of shrinkage of such shrink overwrap films must be controlled to avoid damaging the wrapped articles due the film's shrinkage force.
An additional intended property of typical overwrap films, of both the stretch and shrink types, is adhesion of the film to itself and/or to the article being wrapped. Generally, the lower the density of the polyethylene, the tackier or stickier the material. Thus, one method for adhering the film is by self-adhesion, where one surface of the film is tacky and is wrapped onto and adhered to itself. Another method of adhering the film to itself, especially suitable for shrink films, is by overlapping ends of the film and heat-sealing, effectively melting the overlapped portions to each other, and cooling to create a weld.
Overwrap films are often designed to have properties dependent upon the particular customer or end-user. To achieve these properties, the film may be coated with an adjuvant to create a sticky outer layer or a slippery outer layer. A different method of altering the various surface properties of the film is to create a multilayer film wherein a core layer, having properties desired for the internal structure of the film, is sandwiched between skin layers, having the desired properties for the exterior of the film. One of the problems with these multilayer structures is compatibility of the various layers, which can impair the integrity if the unitary multilayer structure. To avoid separation or delamination of the layers, multifarious approaches, including adhesives and bi-adherent (tie) layers between the various structure layers, can be used to keep the structural layers of the multilayer film together as an integral film.
One of the more important active properties of certain films is the inhibition of corrosion, mainly oxidation, of the packed item(s) by gases sealed in with the item(s) packed and /or by diffusion through the film; of course, there is also the common occurrence of insults (pokes and tears) in the film after the item is packed (e.g., during handling and/or transportation). The most common "corrosive" gases found are oxygen and water vapor. Water is a universal solvent and causes or catalyzes oxidation of most metal and other surfaces (with respect to the bulk material, a surface generally has broken bonds available for oxidation). For example, when an item having exposed metal surfaces is wrapped in a stretch film, water vapor trapped in the wrapping, or entering through a tear, or concentrated due to a fall in the environmental temperature (e.g., water vapor having condensed on a wrapped item stored on a cold loading dock) will start to degrade the exposed metal surfaces. This degradation is typically referred to as "vapor corrosion" and products and compositions for its prevention are typically called "vapor corrosion inhibitors" or "VCIs" in the industry. VCI technology include hygroscopic compounds applied to and/or devices sealed in with the items, as well as certain types of films and tapes having hygroscopic and/or protective compounds that are emitted from the film or tape. For example, VCI products such as a protective spray, a plastic and a foam emitter, an impregnated foam tape, a high density polyethylene (HDPE) film, a linear low density stretch film, coated papers, breathable pouches, and the like; such are also described in U.S. Pat. No. 5,422,187 (and those of related applications, such as U.S. Pat No. 5,139,700, to Cortec, St. Paul. Minn., the disclosures of which are all incorporated herein by reference).
In spite of the foregoing, there remains a need for stretch wrapping films having improved properties; namely, improved stretch levels, puncture resistance, tensile strength, tear strength, a higher holding force, and desired cling properties. In addition, there are significant problems incorporating both a VCI and a tackifier into the same film. One problem is that tackifiers are typically viscous liquid concentrates that are added to the melt composition before extrusion, and as such are messy and difficult to handle. Another problem is that the VCI can block the migration of the tackifier to the surface, thus effectively reducing the cling properties of the film. Also, it is desirable to process certain VCIs at relatively low extrusion temperatures, such as below 400.degree. F., preferably below about 370.degree. F., to avoid degrading the VCI composition.